1. Field of the Invention
The present invention relates to ink jet printing and more particularly, to an improved droplet interlace procedure for enhancing drop placement accuracy in such printing.
2. Prior Art
Ink jet printers wherein ink droplets impinge on a recording medium moving past the printer are known. A typical ink jet printer comprises a droplet generator which selectively and in a controlled manner causes ink droplets to strike a recording medium such as paper, thereby providing a permanent record of the information. Ink jet printers can comprise a single nozzle through which the droplets are directed to a relatively moving recording member or alternatively, a number of nozzles can simultaneously direct ink droplets to the medium. Ink jet printers are used in word processing as well as graphic display applications and have perceived speed capabilities in excess of conventional impact type printers.
A number of ink jet printing architectures have evolved. One type of ink jet printer uses the so-called drop-on-demand printing technique. In a drop-on-demand system, droplets are generated for only those incremental areas on the medium where information is to be encoded. Each droplet follows a straight path to the paper. No complex droplet charging or guttering mechanism is required, since all drops which are emitted from the droplet generator strike the paper.
One perceived constraint on the drop-on-demand configuration is an upper boundary to the speed of information throughput such a system can handle. If, for example, the ink jet system is to be employed in a letter quality printer, it is presently believed a copy rate of about one page every 30 seconds is possible with the drop-on-demand system. While this speed may be adequate for a typewriter, it is not adequate for high speed duplicating or copying. For those ink jet applications requiring high speed operation, a second type of architecture utilizing continuous drop production is preferred.
U.S. Pat. No. 3,596,275 to Sweet discloses a recording system wherein a sequence of ink drops are directed to a recording medium in a controlled manner in order to encode that medium with information. A typical Sweet type ink printer has one or more ink jet nozzles through which ink under pressure is directed toward a record medium such as paper. As ink is forced through the one or more nozzles, an external source of energy provides a perturbation to the ink to induce droplets to break off at controlled intervals a well-defined distance from the ink droplet generator. At the point of droplet break off, these droplets may be immediately charged by induction so that the droplet trajectory may be altered or modified by a uniform electric field downstream from the droplet formation point.
Certain subclasses exist in the continuous type ink jet printing architecture. In a so-called binary arrangement, the ink droplets either travel in a path to a guttering system where the ink is recirculated back to the ink jet generator or in the alternative, travel along a path to the record medium. The alternate droplet trajectories are achieved by selectively charging the ink droplets by a technique known in the art. As in the drop-on-demand architecture, this type of ink jet printing can be performed with either single or multiple ink jet nozzles.
A second type of continuous droplet formation system employs a transverse scanning architecture wherein once droplets have been charged to an appropriate value, passage through a uniform electric field interposed between the generator and the record medium causes the ink droplet to scan from side-to-side in the direction transverse to paper motion. In this so-called "stitched" arrangement, a given ink jet nozzle supplies ink droplets to a number of incremental areas (pixels) on the paper. The term "stitched" derives from the fact that ink droplets from adjacent nozzles must be carefully positioned on the recording medium so that they stitch together to completely cover the width of the paper. It should be appreciated that for both a stitched type and binary type continuous droplet arrangement, relative longitudinal movement between the generator and the paper is provided as ink droplets fly towards the paper.
An example of a side-to-side scanning ink jet arrangement is disclosed in co-pending U.S. patent application entitled "Bipolar Ink Jet Method and Apparatus", Ser. No. 296,922, filed Apr. 27, 1981 to A. Lipani et al which is incorporated herein by reference. The apparatus constructed in conformity with the invention disclosed in that application includes an ink jet marking array having a number of ink jet column generators for directing a series of ink droplets in the direction of a recording medium. A series of spaced electrodes are utilized for creating regions of substantially uniform electric field strength through which the ink droplets travel in their trajectory towards the recording medium. Prior to entering the region of electric field generated by the plurality of electrodes, a charging mechanism induces charge on the droplets thereby causing those droplets to experience a desired deflection as they pass through the electric field generated by the electrodes.
The architecture disclosed in the aforementioned patent application utilizes a bipolar scanning technique as a preferred embodiment, but, of course, would be also applicable in an ink jet device utilizing an induced charge of a single polarity for the droplets. In the preferred bipolar arrangement, droplets are either positively or negatively charged depending upon the direction of intended deflection once they enter the electric field. As documented in the foregoing patent appliction, this bipolar arrangement provides one strategy in that it helps limit the time of flight of the droplets in their travel to the recording medium.
A second strategy disclosed in the aforementioned application is the utilization of an interlace charging strategy which specially separates charged ink droplets thereby reducing interactions between droplets as they travel to their intended positions on the recording medium. The aim of the interlace strategy is to spacially separate sequentially generated ink droplets which otherwise would adversely interact with each other due to their close proximity in space. More particularly, as noted in the Lipani et al application, aerodynamic and electrostatic interactions between closely positioned successive ink droplets can misposition those droplets in flight and result in drop placement errors.
The theory of an interlace charging scheme is to spread out the position of the charged droplets in their flight path to the recording medium so that each droplet experiences a uniform aerodynamic breaking effect and also to reduce the coulomb attractions and repulsions between closely adjacent droplets. It should be recalled that in a stitched, continuous droplet-production system, each nozzle generates a number of ink droplets which sequentially scan across the width of a portion of the paper as the image or information is recorded on the receiving member. The interlace strategy disrupts or interrupts the sequential scanning of ink droplets so that successive droplets strike non-successive picture elements (pixels) on a given recording member portion. Thus, in scanning across a portion of the receiving member, adjacent picture elements will be printed by non-successive ink droplets. The advantages of such a system are documented in the Lipani et al application and will be reviewed in the present discussion of a preferred embodiment of the invention.
In a typical interlace or non-sequential printing pattern, as the interlace level is increased, the axial or drop-to-drop separation along the direction of droplet movement increases as the transverse or side-to-side droplet separation is reduced. If, for example, a quadrupole interlace scheme is envisioned (i.e. the number of adjacent linear pixel locations on the recording medium allotted to the droplets of one of a plurality of nozzles is divided into four portions), the axial separation between droplets is on the order of 4 droplet spacings where the droplet spacing equals the droplet velocity divided by the frequency of droplet production. In a prior art quadrupole interlace scheme, however, sequentially generated droplets are transversely separated in the direction of droplet deflection no more than one-quarter the width of the channel through which a particular nozzle sends droplets.
In a non-interlace scheme shown in FIG. 2A, the first droplet is directed to the first pixel location, the second droplet is directed to the second location, and the third droplet is directed to the third pixel location, etc. As can be seen in FIG. 2A, the axial spacing (y) is the distance between adjacently generated droplets; for example, the distance between the first and second droplets. The maximum transverse spacing (x) is the distance between adjacent pixel locations.
A triple interlace scheme is depicted in FIG. 2B wherein a lineal number of adjacent pixel locations alloted to one nozzles is 42 and this number of pixel locations is divided into three segments of fourteen pixels each. With this prior art scheme, sequentially generated droplets are serially placed one each in the fourteen pixel segments. The first droplet is directed to pixel location number 1, the next generated droplet is directed to pixel location number 15 which is the first pixel location in the second segment, the third generated droplet is directed to pixel location number 29 which is the first pixel location in the third segment, the fourth generated droplet is directed to the pixel location 2 which is the second pixel location in the first segment, etc. As can be seen, one droplet, sequentially generated, goes to each segment in ascending order of pixel locations before repeating the sequence for the pixel locations adjacent the ones first addressed. The targeted pixel location for the droplet, numbered in order that they are generated, are placed in parenthesis beside the droplet number. As the order or number of interlace is increased to accommodate a desire for greater axial separation, the side-to-side separation for sequentially generated droplets decreases.
It is one aim of the present invention to allow increased axial separation between droplets while maintaining the side-to-side or transverse separation to avoid droplet misplacement caused by coulomb interaction between droplets.